Guns didn't disappear, they were hoarded by the new governments and banned to everyone else. Also, it's rather difficult to make the components of modern ammo without electricity so that's also become hard to come by.

Never thought about the bicycle thing so you make a point there...

Uhh, I've got news for you.....There is an untold amount of ammo in circulation, hidden away, etc and people have been shooting guns long before there was electricity.